1. Field of the Invention
This invention relates to a high voltage generator which boots flyback pulses (collector pulses) so as to obtain a high voltage to be supplied to an anode of a cathode ray tube.
2. Description of the Related Art
Usually, a high voltage generator supplies a high voltage in the order of tens of kilovolts, to an anode of a cathode ray tube in a television receiver, a display, or the like. The high voltage generator obtains such a high voltage by boosting and rectifying flyback pulses generated in a horizontal output circuit. A flyback transformer is used to boost the flyback pulses.
There is currently available a high voltage generator which includes a deflection yoke unit at a low-voltage coil side of the flyback transformer. In this arrangement, flyback pulses are used to obtain a sawtooth deflection current to be applied to the deflection yoke unit.
Such a high voltage generator often suffers from a problem that correction of the high output voltage adversely affects the operation of a circuit in the deflection yoke unit. Specifically, a voltage is applied to compensate for a drop in the high voltage so as to stabilize the high voltage, which adversely affects the deflection yoke circuit.
Recently, there has been proposed a circuit configuration in which the high voltage generator and the deflection yoke circuit are independent of each other so as to prevent interference between them. In such a configuration, the flyback pulses are generated in synchronization with the period of a horizontal deflection current from a horizontal output circuit.
FIG. 16 of the accompanying drawings shows one example of the foregoing high voltage generators. This circuit is disclosed in Japanese Patent Laid-Open Publication Hei 2-222374 corresponding to U.S. Pat. No. 5,019,953, and generates flyback pulses in synchronization with horizontal drive signals HD of a horizontal output circuit, rectifying the flyback pulses so as to obtain a high output voltage.
In the circuit, a peak value of the flyback pulses boosted by the flyback transformer is controlled by controlling the on-time of a transistor 1. Assume that an on-time 1a is changed to an on-time 1b (as shown in FIG. 17(b)), i.e. the on-time 1a is lengthened. In this case, a collector current of an output transistor 4 flowing during an off-time of the transistor 1 (i.e. a closed loop current flowing via a diode 2, a low-voltage coil of the flyback transformer and the output transistor 4 and returning to the diode 2) becomes large as shown in FIG. 17(c), so that the peak value of the flyback pulses applied to the low-voltage coil also becomes large as shown in FIG. 17(a). On the contrary, when the on-time of the transistor 1 is shortened from 1a to 1c as shown in FIG. 17(b), a small collector current flows via the transistor 4 during the off-time of the transistor 1 (as shown in FIG. 17(c)), thereby leading to a reduced peak value of the flyback pulses (as shown in FIG. 17(a)).
Therefore, the peak value of the flyback (collector) pulses depends upon the on-period of the pulse signal to be applied to the base of the transistor 1. The on-period of the pulse signal is determined by the timing to turn off the transistor 1.
As described above, the high voltage generator boosts the collector pulses in the flyback transformer, and rectifies them in a rectifier so as to obtain a high output voltage. The generated high output voltage is detected by a high output voltage detecting circuit. Then, the on-time of the pulse control signal is controlled based on the detected results, thereby stabilizing the high voltage by compensating for a drop therein.
However, a large amount energy is required for the flyback operation, which means that a large collector current flows through the transistor 4. The large collector current is consumed by element such as the diode 2, the low-voltage coil of the flyback transformer and the transistor 4 in the closed loop and is transformed to the heat. Therefore, this closed loop current may be one of the factors which reduces the circuit efficiency.
The transistor 1 is switched on and off during the scanning operation of the television receiver or display unit. Noises caused by the switching of the transistor 1 may distort a raster appearing on the screen of television receiver or display unit.
When compensating for the voltage drop, the generator circuit conventionally operates to reduce the output voltage as compared with the voltage before the compensation, so that an input voltage at a power source +B and a boost ratio of the flyback transformer have to be large. This means that electronic components in the generator circuit are subject to a large load.